gcc7: fix native and cross-musl build, sync w/gcc6 fixes

pkgs/development/compilers/gcc/7/default.nix

@@ -143,6 +143,9 @@ let version = "7.3.0";
         "--disable-shared"
         "--disable-libatomic"  # libatomic requires libc
         "--disable-decimal-float" # libdecnumber requires libc
+        # maybe only needed on musl, PATH_MAX
+        # https://github.com/richfelker/musl-cross-make/blob/0867cdf300618d1e3e87a0a939fa4427207ad9d7/litecross/Makefile#L62
+        "--disable-libmpx"
       ] else [
         (if crossDarwin then "--with-sysroot=${getLib libcCross}/share/sysroot"
          else                "--with-headers=${getDev libcCross}/include")
@@ -166,6 +169,8 @@ let version = "7.3.0";
             # In uclibc cases, libgomp needs an additional '-ldl'
             # and as I don't know how to pass it, I disable libgomp.
             "--disable-libgomp"
+            # musl at least, disable: https://git.buildroot.net/buildroot/commit/?id=873d4019f7fb00f6a80592224236b3ba7d657865
+            "--disable-libmpx"
           ] ++ [
           "--enable-threads=posix"
           "--enable-nls"
@@ -268,7 +273,7 @@ stdenv.mkDerivation ({
                  -e 's|define[[:blank:]]*MUSL_DYNAMIC_LINKER\([0-9]*\)[[:blank:]]"\([^\"]\+\)"$|define MUSL_DYNAMIC_LINKER\1 "${libc.out}\2"|g'
            done
         ''
-        + stdenv.lib.optionalString (hostPlatform.libc == "musl")
+        + stdenv.lib.optionalString (targetPlatform.libc == "musl")
         ''
             sed -i gcc/config/linux.h -e '1i#undef LOCAL_INCLUDE_DIR'
         ''